Field of the Invention
The present invention relates to the field of renewable energy and more particularly, to the measurement of the parameters of the wind turbines, that is the wind speed and turbine control objectives of orientation, torque and speed regulation.
Description of the Prior Art
A wind turbine allows the kinetic energy from the wind to be converted into electrical or mechanical energy. For conversion of the energy from the wind to electrical energy, the wind turbine includes the following elements:                (a) A tower positioning a rotor at a sufficient height to enable motion thereof (necessary for horizontal-axis wind turbines) or for the rotor to be positioned at a height enabling the turbine to be driven by a stronger and more regular wind than at ground level. The tower generally houses part of the electrical and electronic components (modulator, control, multiplier, generator, etc.);        (b) A nacelle mounted at the top of the tower which houses mechanical, pneumatic and some electrical and electronic components which are necessary to operate the machine. The nacelle can rotate to adjust the machine to the right direction to be driven by the wind;        (c) A rotor fastened to the nacelle comprising several blades (generally three) and the nose of the wind turbine. The rotor is driven by wind power and it is connected by a mechanical shaft, directly or indirectly (via a gearbox and mechanical shaft system), to an electrical machine (electrical generator) that converts the recovered energy into electrical energy. The rotor may be provided with control systems such as variable-angle blades or aerodynamic brakes; and        (d) A transmission having two shafts (mechanical shaft of the rotor and mechanical shaft of the electric machine) connected by a transmission (gearbox).        
Since the beginning of the 1990s, renewed interest has occurred in wind power, in particular in the European Union where the annual growth rate is about 20%. This growth is attributed to obtaining carbon-emission-free electricity generation. In order to sustain this growth, the energy yield of wind turbines has to be improved. The prospect of wind power production increase requires developing effective production tools and advanced control tools in order to improve the performances of the machines. Wind turbines are designed to produce electricity at the lowest possible cost. They are therefore generally built to reach their maximum performance at a wind speed of approximately 15 m/s. It is not necessary to design wind turbines that maximize their yield at higher wind speeds, which are not common. In case of wind speeds above 15 m/s, part of the additional energy contained in the wind is lost to avoid damage to the wind turbine. All wind turbines are therefore designed with a power regulation system.
For this power regulation, controllers have been designed for variable-speed wind turbines. The purpose of the controllers is to maximize the electrical power which is recovered, to minimize the rotor speed fluctuations and to minimize the fatigue and the extreme moments of the structure (blades, tower and platform).
To optimize control, it is important to know the wind speed at the rotor of the wind turbine. Various techniques have been developed to that end.
According to a first technique, using an anemometer allows estimating a wind speed at one point, but this imprecise technology does not enable measurement of an entire wind field or to know the three-dimensional components of the wind speed.
According to a second technique, a LIDAR (Light Detection And Ranging) sensor can be used. LIDAR is a remote sensing or optical measurement technology based on the analysis of the properties of a beam returned toward the transmitter. This method is notably used for determining the distance to an object by means of a pulse laser. Unlike radars based on a similar principle, LIDAR sensors use visible or infrared light instead of radio waves. The distance to an object or a surface is given by the measurement of the delay between the pulse and the detection of the reflected signal.
In the field of wind turbines, LIDAR sensors are described as essential for proper functioning of large wind turbines, especially now that their size and power increases (today 5 MW, soon 10 MW). This sensor allows remote wind measurements which first allow wind turbines to be calibrated so that they can deliver maximum power (power curve optimization). For this calibration stage, the sensor can be positioned on the ground and vertically oriented (profiler), which allows measurement of the wind speed and direction, as well as the wind gradient depending on the altitude. This application is particularly critical because it allows knowing the energy generating resource. This is important for wind turbine projects since it conditions the financial viability of the project.
A second application sets the sensor on the nacelle of the wind turbine for measuring the wind field in front of the turbine while being nearly horizontally oriented. A priori, measuring the wind field in front of the turbine allows knowing in advance the turbulence the wind turbine is going to encounter. However, current wind turbine control and monitoring techniques do not allow measurement performed by a LIDAR sensor for estimating precisely the speed of the wind at the rotor.